docs: fix a Sphinx deprecation warning

 * Handling of javascript mods for the html changed, wasn't going to
   work in 3.0. Basically copied fix from
   readthedocs/sphinx_rtd_theme#728
 * This was used for the "hide prompts" feature in example code.
   New version still works in 1.6, but no longer in 1.5 or earlier.
   Docs still render fine, just don't have the "hide" button, so this
   is okay.

docs/_templates/layout.html

@@ -1,6 +1,10 @@
 {% extends "!layout.html" %}
 
-{% set script_files = script_files + ["_static/copybutton.js"] %}
+{%- block scripts %}
+    {{ super() }}
+    <script type="text/javascript" src="{{ pathto('_static/copybutton.js', 1) }}"></script>
+
+{%- endblock %}
 
 {% block rootrellink %}
         <li><a href="{{ pathto('search') }}">search</a>|&nbsp;</li>